It is well known to produce aromatic compounds such as benzene, toluene and xylenes from petroleum naphtha streams. Attempts have also been made to produce useful aromatic compounds from low molecular weight aliphatic compounds by, for example, the pyrolysis of natural gas, acetylene and other gases. However, this technique produces benzene and other useful aromatic compounds in very low yields while producing large amounts of tar, insoluble carbon residue and high molecular weight aromatic compounds, all of which are of little commercial use. Specifically, in the pyrolysis of methane and acetylene, the reaction is carried out at a temperature of about 1,000.degree. C. or higher with a conversion rate of only a few percent and a selectivity to naphthalenes of less than 1%. Consequently, this method has little practical application.
There are reports in the art of processes for converting natural gas into aromatic compounds. For example, U.S. Pat. No. 5,288,935 discloses a process for producing liquid hydrocarbons from natural gas, in which natural gas is first separated into a methane rich fraction and a C.sub.2 + fraction, the methane is then selectively oxidized with oxygen, the effluent from the selective oxidation is then mixed with a part of the C.sub.2 + fraction and pyrolyzing the resulting mixture to obtain an aromatic product. The final step is carried out at a temperature of about 300.degree. C. to about 750.degree. C. in the presence of an aromatizing catalyst consisting essentially of a zeolite, gallium, at least one metal from the Group VIII metals and rhenium and at least one additional metal selected from the group consisting of: tin, germanium, lead, indium, thallium, copper, gold, nickel, iron, chromium, molybdenum and tungsten; an alkaline metal or alkaline earth metal and an aluminum matrix.
It is also known that the dehydrocondensation of methane with CO or CO.sub.2 to form benzene and naphthalene can be carried out using a molybdenum/HZSM-5 or iron/cobalt modified Mo/HZSM-5. S. Liu, Q. Dong, R. Ohonishi and M. Ichikawa, Chem. Commun. (1998), p. 1217-1218, and S. Liu, L. Wang, Q. Dong, R. Ohonishi, and M. Ichikawa, Stud. Surf. Sci. Catal., Vol. 119, p. 241-246. In contrast to this art, applicants have developed a novel catalyst which comprises rhenium on a porous support such as a zeolite and which optionally can contain other metals such as iron, cobalt, platinum and molybdenum. It has been found that the catalysts of the present invention have higher activities for converting methane to benzene and also have a higher selectivity for the higher carbon number hydrocarbon products such as benzene, toluene and xylene and ethane and ethylene.